1. Field of the Invention
The present invention relates to a motor driving apparatus and a motor driving method which enable start of a motor without use of a sensor for detecting a rotor position.
2. Related Art
In driving of a brushless motor, it is necessary to obtain an electrical relative position of a rotor to a stator in order to select an appropriate phase of the stator for supplying a current to apply a stable torque to the rotor. A variety of rotor position sensors are used for obtaining an electrical relative position of the rotor to the stator. Meanwhile, a sensorless driving technique which requires no rotor position sensor is under development from the viewpoint of reliability, an increase in cost, and environmental resistance.
In such a sensorless driving technique, it is generally known that a rotor position is detected by reading a back electromotive voltage generated in the motor coil during rotation of the rotor. However, the back electromotive voltage is not generated during stoppage of the rotor, and hence a variety of methods have been proposed as rotor position detecting methods during stoppage of the rotor.
For example, Japanese Patent No. 2547778 shows a method of applying an initial position detection pulse as a pulse for detecting an initial position of a rotor, while sequentially selecting phases of a stator, to detect a rotor position from a phase where a current of a maximum value is flowing at the moment.
Further, in JP-A-07-83628, a rotor position is obtained in the following manner. Phases of a stator are sequentially selected, and a rotor position search pulse is applied thereto. At that time, voltages that appear at a neutral point of motor coils are divided into a first measured voltage group showing measured voltage values close to a third of a power supply voltage value and a second measured voltage group showing measured voltage values close to two thirds of the power supply voltage, and each of the voltage values are then stored. From the storage data of each measured value, a difference between the minimum absolute voltage value and the maximum absolute voltage value of each of the voltage groups is obtained. The obtained difference voltages of the respective groups are compared with each other, and the rotor position is determined on the basis of a conduction pattern in which a larger difference voltage is obtained.
Further, according to a motor driving apparatus disclosed in JP-A-2004-104846, phases of a stator are sequentially selected and a rotor position search pulse is applied to each selected phase. A neutral point voltage of motor coils, which is generated in flowing of the rotor position search pulse current, is inputted into a detection circuit for detecting a rotor position. As thus described, the method is shown for determining the rotor position on the basis of reference data of the neutral point voltage for rotor position determination. Further, in the detection circuit, a detection level is shifted to a level according to a drive signal for rotor position detection, and comparison by a comparator between the shifted detection level and the neutral point voltage of the motor coils is outputted and then stored, to determine the rotor position on the basis of the comparison between the stored data and reference data of variations in neutral point voltage for rotor position determination.
Since in the brushless motor, vibration, noise and rotational variations have to be suppressed by devising structure of the motor, a variety of motors with magnets of rotors magnetized by varied methods, cores in varied shapes, or other elements varied, have appeared in the market. A motor driving apparatus for driving such motors is required to reliably start the motors.
In the above-mentioned conventional method, the mode is shifted from the rotor position searching process to an operational mode for driving the motor while detecting a back electromotive voltage (hereinafter referred to as “back electromotive voltage mode”). Therefore, when the initial position has not been successfully detected in the rotor position searching process, the motor remains stopped. Moreover, when the detected initial position is inaccurate, appropriate initial acceleration has not been given to the motor to shift the mode to the back electromotive voltage detection mode, which might cause occurrence of erroneous operation (reversal etc.) due to starting failure.